Recently, development of communication networks and in particular of radio communication networks has made considerable progress. For example, the GSM network and its successors such as the GPRS and/or UMTS networks find increasing attention.
Basically, radio communication networks are constituted by transceiver devices also known as base transceiver stations BTS. A mobile transceiver station also known as mobile station MS (according to GSM) and/or user equipment UE (according to UMTS) communicates via the radio interface (air interface) with the transceiver devices constituting the network. Of course, the transceiver devices are controlled by other network elements of a higher hierarchy which, however, are omitted from the description in this case as this is considered to be not essential to the invention to be described.
Generally, a mobile station MS may move and/or roam within the radio communication network. In this connection, there may arise a situation in which a mobile station MS leaves the coverage area of a serving base transceiver station BTS and is handed over to a new base transceiver station BTS.
To this end, a knowledge of the transceiver stations surrounding the mobile station and/or a serving transceiver station is required in order to take a proper decision on where to handover the moving mobile station. Correspondingly, a knowledge of the mobile station's position within the network is required.
According to one principle, the required knowledge is obtained by measurements performed by e.g. the mobile station itself, which measures the observed time difference OTD between signals received at the mobile station from a pair of transceiver stations BTS, e.g. between a currently serving base transceiver station and a respective other base transceiver station.
This is known as OTD method and described in literature in detail. Briefly summarized, e.g. in GSM phase 2 systems, support for pseudo-synchronous handover is compulsory. In a pseudo-synchronous handover, the mobile station MS will keep the timing values for the surrounding base transceiver stations BTSs in order to be pre-synchronized to the new BTS upon handover. To obtain this synchronization, the MS must calculate and/or measure an Observed Time Difference (OTD) between signals received from the serving BTS and the other BTSs. Each BTS must maintain a Real Time Difference, RTD, between itself and a respective one of its neighboring base stations. When handover is performed, the RTD is supplied to the mobile station MS, which with the knowledge of the RTD and OTD can calculate the Timing Advance needed to synchronize with the new BTS, and go directly into synchronization. Further details of this procedure are defined in GSM specification 05.10.
There exists also a modified OTD scheme, which is known as Enhanced Observed Time Difference (E-OTD). In brief, the E-OTD positioning method is based on signal measurements made by the mobile station (MS) and location measurement units (LMUs), which are essentially stationary mobiles of known position. To calculate the position of the mobile, the network uses three parameters: observed time difference (OTD), real time difference (RTD), and geometric time difference (GTD). GTD is derived from OTD and RTD, and gives the mobile's position. The OTD measurements are time intervals measured by a mobile station MS between received signals originating from two different base stations. Because the GSM network is not synchronized, the network measures the RTD, which is the relative synchronization difference between the two base stations. To obtain accurate triangulation, OTD and RTD measurements of at least three geographically distinct base stations are needed. Based on the measured OTD and RTD values, the location of the MS can be calculated. The position of the MS is determined by deducing the geometrical components of the time delays (GTD) to a mobile station MS from the base stations.
E-OTD as a mobile station location method in GSM requires that the mobile is able to receive at least two neighboring base stations (in addition to the currently serving BTS). According to GSM specification 04.31, the mobile station can identify the neighbor BTS by using:    1) an index referring to the BTS listed in the Measure Position Request component,    2) an index referring to the BTS listed in the BCCH allocation list (System Information Neighbor Lists) of the serving BTS,    3) cell identity CI and location area code LAC,    4) base station identity code BSIC value and broadcast control channel BCCH carrier information,    5) 51-Multiframe offset and BCCH carrier. Correct identification of the neighbor base station is of outmost importance for successful and accurate location.
Nevertheless, there may arise situations in which the above listed measures will not be sufficient to properly identify a base transceiver station and/or transceiver device constituting the radio communication network without doubt.
Stated in other words, BSIC and BCCH carrier are not a unique way to identify a base station. Theoretically, a proper network planning should ensure that it is not possible for a mobile station to receive signals from two or more different base stations with the same BSIC and BCCH carrier combination, and that based on a serving base station it is evident, which neighbor base station (with a certain BSIC and BCCH carrier combination) the mobile has measured.
However, in reality it is possible that the same BSIC and BCCH carrier combination repeats itself so tightly (i.e. spatially close to each other within the network) that situations arise in which it is ambiguous (not clear) which neighbor base station was measured. Since mobile stations mainly use the information of BSIC and BCCH carrier for neighbor base station identification, there may arise a problem.    Likewise, when a neighbor base station is identified with a 51-Multiframe offset and BCCH carrier, the same problem can arise if a frequency reuse pattern is tight, and 51-Multiframe offsets happen to be the same.
In some cases it is also possible that two different base stations share the same cell identity CI and location area code LAC. This can happen if Inter-PLMN HO feature is in use (PLMN=Public Land Mobile Network, HO=Handover). Inter PLMN HO feature means basically that a base transceiver station BTS can have neighbor cells which belong to different countries and/or operators and MS can make a handover to those cells, as it is the case e.g. in international roaming near country borders.
FIG. 1 schematically illustrates such a situation. A serving BTS also referred to as reference BTS is not shown in FIG. 1. Rather, there is shown the mobile station MS trying to identify its surrounding base transceiver stations, i.e. potential neighbors BTS1, BTS2 to which e.g. a handover HO could be performed. As shown in FIG. 1, the mobile station MS receives identical information concerning the BSIC for the two base transceiver stations, i.e. BSIC1=BSIC2. Also, the BCCH information for BTS1 and BTS2 is identical (BCCH1=BCCH2), and also the offsets, i.e. 51-multiframe offsets offset1/offset2 are identical. In such a situation, it is evident that identifying said transceiver devices (when e.g. accomplished based on respective broadcast control channel BCCH information of said transceiver devices and/or on a respective base station identity code BSIC of said transceiver devices) leads to an ambiguous result since it is evident to be judged that the transceiver devices can no longer be distinguished from each other.
In addition to an identification problem, there can be situations, when the E-OTD value reported by the mobile station can be corrupted because the mobile station performs for some reason (e.g. bad radio environment) very poor measurements, or because for other reasons the measurement information from the mobile station is bad.